The passenger pigeon is long gone, hunted from the skies for cheap meat. While predation and habitat loss still remain major threats to wildlife, today's extinctions aren't always the result of something so deliberate as a bullet.

A State of the Birds report published last year by government and conservation organizations warned that bird species that have been in a steady decline now face added pressure from climate change. Birds from every habitat--from boreal forests to subtropical islands--will have to adapt to the shifts in their already compromised habitat.